John Sweeney had been a keen sports man. He played Gaelic football for Antrim in his teens and was remarkably fit while still in his early 20s.

But when he took a year out to go travelling in Australia, he gave up the fitness routine too. Gradually his weight crept up and when he returned to Ireland two years ago, he was two stone heavier than he had been before he left. Standing 5ft 10ins, he had gone from a lean 12.5 stone to a flabby 14.5 stone.

Friends and family noticed the change and commented on it. "That was hard," John (28) says. "I didn't realise until that point just how much weight I'd put on or how unfit I'd become. It was a real wake up call."

He joined a well-known gym in Dublin, hoping that he could shift those rogue pounds. "Nothing happened. I was the same weight three months later. The experience was dismal. I'd go in after work and all the treadmills would be taken. I'd have to wait around to use them.

"When I signed up, they promised me that they would check my progress regularly. That never happened."

To make matters worse, John found himself tied into a year-long contract that he couldn't get out of.

And then he discovered Bootcamp Ireland, the outdoor training phenomenon that has grown rapidly in seven years. "I noticed one of their flyers and liked the idea of it," he says.

"My girlfriend got me a voucher for an eight-week course and I went along to [Dublin's] Marlay Park, not sure what to expect. After the first session in a class for beginners/intermediate, I knew it was for me. The cardio and flexibility workouts were tough, but doing it outdoors with a group made it easier.

"The routine was so varied that there was no time to get bored. I started in summer, which made it easier, but after my experience in the gym this was a completely different world."

That was 18 months ago and John has subsequently shed 2.5 stone. "I noticed a change after just two weeks and that spurred me on to throw myself into the routines. I'm fitter than I've ever been, more so even than when I played inter-county football. I go in the evenings twice a week and also every Saturday morning. I'm in the advanced classes and they're difficult. You've got to work hard to keep up or the instructor might tell you to go back to the beginners class."

John says Bootcamp Ireland has transformed his life. Never much interested in running, he has completed two marathons and is so taken now that he is training to become an instructor.

He hopes to leave his job in the pensions industry next year and join the fitness business full time. "I'm zealous about it," he says.

Norma O'Kelly's life has also changed beyond recognition. The 32-year-old telecommunications project manager from Limerick used to be "gym adverse", as she puts it, but thanks to dedicated outdoor fitness sessions has become a regular runner.

"If you had told me a few years ago that I would be training to run a half-marathon, I'd have thought you were mad," she says. "I thought running that sort of distance wasn't something I'd ever be able to do."

Like John, Norma also had an unhappy experience with gyms. "I hated them. I think they work for some people, but I always considered it a chore, rather than something enjoyable and when you don't see yourself getting any fitter, it's hard to take."

It was only when she noticed a group of women exercising on Sandymount Strand, Dublin, that she realised there were compelling alternatives to conventional gyms.

"They were all ages and all shapes and sizes. It looked like fun, even though it was obvious that they were being put through their paces."

The group she saw was part of a comparatively new fitness venture, Forget the Gym, and Norma -- along with some friends -- signed up. She hasn't looked back.

"It made me think about fitness in a whole new light," she says. "There's something really nice about working out in the open air and there was a sense of camaraderie from the off. We do running exercises and pilates and after each session you feel really worked out."

Norma noticed results straight away. Within a few weeks she had dropped a dress size and after six weeks she had gone from struggling to run any distance to comfortably tackling five kilometres.

The brainchild of Dublin-based Meath woman Mary Jennings, Forget the Gym has been operating for four years and aims to get members off the couch and into 5k runs in just eight weeks.

"I've been a member of gyms but they didn't do much for me," she says. "Like a lot of people in the Celtic Tiger years, it seemed to be more about being a member of a gym rather than using it properly."

A friend introduced her to jogging and so taken was she with her rising fitness and general well-being that she secured the necessary qualifications (everything from sports massage to chi running and Nordic walking), packed in her marketing job and set up Forget the Gym.

"I realised there were a lot of people like me who wanted to get fit but didn't get anything from conventional gyms.

"One of the things that appeals to clients is the fact that they don't have to sign their life away -- it costs as little as €90 to join for two months. There are no hidden charges."

One might be forgiven for assuming that Ireland's winter climate is hardly the most suitable for outdoor training, but Mary believes otherwise.

"We're out come rain, hail or sunshine," she says. "Numbers were down during the snow before Christmas, but a lot of people still came out and enjoyed it. Running on snow is okay; obviously ice is to be avoided."

Limerick-born, Dublin raised Lorraine Ho established Bootcamp Ireland in 2003. "Most people won't work out if they're not enjoying it," she says. "I wanted to show that there was an alternative to the gym, where you could get what's effectively personal training but without the huge expense. I had been a gym member before, but I found working out in a group in a park to be so much more enjoyable."

Many of Bootcamp Ireland's instructors are from a military background and are tough taskmasters.

"Clients are pushed to get the most out of their workout," Lorraine says. "The banter is tongue-in-cheek -- nobody is humiliated -- but the idea is to put those who attend through their paces."

There's a strong social aspect to Bootcamp too. "You get to meet people in a way that might be difficult in a gym. Headphones are not allowed and interaction is encouraged. It's much easier to get fitter if you're working out with others than if you're doing it alone."

John Sweeney is certainly converted. "I shudder to think what weight I'd be now if I hadn't joined," he says.

"Sometimes, you need to kick-start a new lifestyle and Bootcamp was that catalyst for me.

"It's so much part of my life now that if I can't make a session I really miss it. That would have been unthinkable in my gym days when I was grateful for any excuse not to go."